Steam

The article also says

The first point is one we've heard repeated many times before, but there's never been any proof on it. Which perhaps the Wolfire lawsuit and this may actually bring to light. An accusation doesn't necessarily mean they're right though. Something people get confused on often is Steam Keys, which are completely separate to Steam Store purchases.

Saying “Don’t sell Steam keys off-platform for more than X% less than the game is priced for on Steam” and “Don’t sell your game elsewhere for more than X% less than the game is priced for on Steam“ are very different things. Steam openly does the former; I’ve never heard a reputable report of them doing the latter. The Wolfire lawsuit is explicitly about the former practice, for example.

The press release for this lawsuit reads like it’s about the latter, but I suspect that’s solely for optics. I reviewed the website dedicated to the lawsuit (steamyouoweus.co.uk) and thought they might have some more concrete evidence - nope, nothing. Under the first question in FAQs they have a link to their key documents, but the documents are “coming soon.”

Until they actually substantiate their claim, this lawsuit is just noise.